The Xdebug extension
Zend extension, and not a PHP extension
Xdebug is very PHP-version sensitive
Different compilers under Windows
Debug/non-debug
Threadsafe or not
In PHP 5.1 and 5.2:
zend_extension=/local/php/lib/php/extensions/no-debug-non-zts-20090626/xdebug.so
zend_extension_ts=/local/php/lib/php/extensions/debug-zts-20090626/xdebug.so
zend_extension_debug=/local/php/lib/php/extensions/debug-non-zts-20090626/xdebug.so
In PHP 5.3:
zend_extension=/local/php/lib/php/extensions/no-debug-non-zts-20090626/xdebug.so
zend_extension=/local/php/lib/php/extensions/debug-zts-20090626/xdebug.so
zend_extension=/local/php/lib/php/extensions/debug-non-zts-20090626/xdebug.so